1. Field of the Invention
The present invention relates to a wavelength locking method of causing a reception wavelength of an optical receiver in wavelength division multiplexing communication networks or the like to track a wavelength of a light source, a wavelength locking apparatus for performing the wavelength locking method, and a wavelength division multiplexing communication network using this apparatus.
2. Related Background Art
Wavelength division multiplexing communication networks generally have a large number of independent channels in a single transmission line. Therefore, in wavelength division multiplexing communication, transmission speeds of the respective channels need not necessarily be equal to each other since multiplexing on a time axis, such as a frame synchronization, is not required. As a result, information different in their qualities, such as video data and audio data, can be collectively treated in a combined manner. Thus, such communications can be suitably employed for multimedia communications for which network flexibility is required.
As an example of the wavelength division multiplexing communication network, there exists a system in which each terminal includes a set of a tunable optical transmitter and a tunable optical receiver. The physical topology of the system is often a star type. Here, each terminal is connected to a star coupler through an optical fiber, and an optical signal from the optical transmitter in each terminal is distributed by the star coupler to the optical receivers in the respective terminals including the transmitting terminal itself. In this state, the transmitting terminal controls a tunable light source in the transmitter such that its wavelength is coincident with a unused wavelength (or a vacant channel), and the receiving terminal controls a peak wavelength of a transmission spectrum of a tunable filter (also referred to as a wavelength of the tunable filter in the specification and claims) in the receiver such that the wavelength becomes equal to the reception wavelength. The receiving terminal thus receives the optical signal. A wavelength range usable in the system is determined by wavelength changeable or tunable ranges of the transmitter and the receiver, and a wavelength spacing between the channels is determined by a width of the transmission spectrum of the tunable filter in the receiver. The wavelength spacing between the channels can be decreased as the width of the transmission spectrum is narrowed.
As the tunable light source in the transmitter, a tunable semiconductor laser (the semiconductor laser is also referred to as LD) can be used. In order for the system to use a wide wavelength range, research has been made to widen a tunable width of the LD. Semiconductor lasers of distributed Bragg reflector (DBR) and distributed feedback (DFB) types are presently used as a practical LD. Their tunable widths are several nanometers or so. As its example, there exists one that is disclosed in "Long Cavity .lambda./4 Shifted MQW-DFB Laser with Three Electrodes", Technical Report OQE (Optical and Quantum Electronics) 89-116, pp.61-66, Electronics Information Communication Association of Japan.
On the other hand, a filter of a Fabry-Perot cavity type can be used as the tunable filter. The filter with a tunable width of several tens nanometers and a transmission spectrum width of about 0.1 nm is considered as a practical one. As its example, there exists one that is disclosed in "A Field-Worthy, High-Performance, Tunable Fiber Fabry-Perot Filter", Conference Paper ECOC (European Conference on Optical Communication), '90-605, '90-608. The fiber Fabry-Perot filter is also referred to as FFP.
As discussed above, the wavelength range usable in the system is determined by the tunable widths of the transmitter and the receiver, and the wavelength spacing between the channels can be lowered as the spectrum width of the tunable filter is narrowed. In addition, where the wavelength spacing between the channels is small, a larger number of channels can be provided even though the wavelength tunable range, which determines the wavelength range usable in the system, is the same. Here, it should be noted that in order for the wavelength spacing between the channels to be smaller than variation or fluctuation due to wavelength drifts of the tunable LD and the tunable filter, causes of the drifts must be suppressed. For this purpose, wavelength controls of the optical receiver and the optical receiver are performed.
As a wavelength control system of the optical transmitter, there exists a system disclosed in Japanese Patent Application Laid-Open 8-163092. In the system, the transmission wavelength of each optical transmitter is placed at a predetermined wavelength spacing .DELTA. .lambda. on a longer wavelength side (or a shorter wavelength side) along a wavelength axis in the order of start of transmission. A state in which the transmission wavelengths are thus positioned at the predetermined wavelength spacing .DELTA. .lambda. along the wavelength axis, is referred to as a stationary state. Therefore, there is no need of providing an absolute or relative wavelength reference in such a system. Each optical transmitter detects a wavelength spacing between the transmission wavelength itself and its adjacent wavelength on a longer wavelength side, and the transmitter feedback-controls its transmission wavelength based on that detection and maintains the transmission wavelength at a position which attains the predetermined wavelength spacing. The detection of the wavelength spacing is conducted by a wavelength scan of a tunable filter provided in the optical transmitter. The optical transmitter, which emits light at a wavelength placed on the longest wavelength side (or the shortest wavelength side) in the wavelength arrangement, maintains the longest wavelength of the tunable LD in the optical transmitter.
A specific operation of the wavelength control in the optical transmitter will be described with reference to FIGS. 1A-1E. Steps illustrated in FIGS. 1A-1E show a procedure during which a terminal starts transmission, the stationary state is reached, another terminal stops transmission and the stationary state is again reached. In FIGS. 1A-1E, .lambda..sub.max and .lambda..sub.min are respectively the longest wavelength end and the shortest wavelength end in the usable wavelength range of the wavelength division multiplexing communication system, .DELTA. .lambda. is the wavelength spacing to be maintained by the control, and .lambda..sub.mar is a margin for the wavelength setting in the tunable LD and the tunable filter (the margin is needed in the system since no calibration of the absolute value of a wavelength is performed). Due to the margin, the actual usable wavelength range is from .lambda..sub.min +.lambda..sub.mar to .lambda..sub.max -.lambda..sub.mar.
In the procedure illustrated in FIGS. 1A-1E, it is assumed that four channels (wavelengths .lambda..sub.c1, .lambda..sub.c2, .lambda..sub.c3, and .lambda..sub.c4) are used before a terminal starts transmission and that the wavelengths are arranged at the spacing of .DELTA. .lambda. starting from .lambda..sub.c1 on the longer wavelength side. The starting point of .lambda..sub.c1 has no adjacent wavelength on its longer wavelength side, so the wavelength .lambda..sub.c1 is at the longest wavelength end in the tunable range of the tunable LD in the optical transmitter that oscillates at this wavelength. However, this wavelength is not always coincident with .lambda..sub.max due to the wavelength setting error, and falls within a range from .lambda..sub.max -.lambda..sub.mar to .lambda..sub.max (see FIG. 1A).
In this state, the terminal, which is going to start emission, starts the emission at the shortest wavelength end after confirming that no other channels are present in a range of .DELTA. .lambda. from the shortest wavelength end (the wavelength is indicated by .lambda..sub.c5 in FIG. 1B). The wavelength .lambda..sub.c5 is present in a range from .lambda..sub.min to .lambda..sub.min +.lambda..sub.mar due to the wavelength setting error (see FIG. 1B). The wavelength .lambda..sub.c5 is shifted toward the longer wavelength side while the terminal confirms no presence of other channels in a range of .DELTA. .lambda. on its longer wavelength side (this step is referred to as an approach step). When the wavelength spacing of .DELTA. .lambda. is reached between .lambda..sub.c5 and .DELTA..sub.c4, the wavelength spacing is maintaned thereafter. Thus the stationary state is attained (see FIG. 1C).
Here, when the terminal which is using the channel of .lambda..sub.c3, stops oscillation, the wavelength spacing between .lambda..sub.c2 and .lambda..sub.c4 comes to 2.times..DELTA. .lambda. and the stationary state is thus lost (see FIG. 1D). Then, the wavelengths .lambda..sub.c4 and .lambda..sub.c5 are gradually shifted toward the longer wavelength side in the same manner as the above approach step. (Here, each optical transmitter is always monitoring if any other channels are present in a range of .DELTA. .lambda. on its longer wavelength side, for example.) When the wavelength spacing of .DELTA. .lambda. is reached between .lambda..sub.c2 and .lambda..sub.c4, the wavelength spacing is maintained thereafter. Thus the stationary state is attained again (see FIG. 1E).
On the other hand, as a wavelength control system in the optical receiver, there is a wavelength locking system or method in which the wavelength of the tunable filter in the optical receiver is locked in or caused to track a selected reception wavelength. The wavelength locking system may perform minute modulation (dither) of the wavelength of the tunable filter and synchronous detection. This locking system will be briefly described.
FIG. 2 shows the structure of a prior art wavelength locking system. The locking system includes an optical power divider 101, a light receiving device 102, an amplifier 103, a sine wave generator 104, a synchronous detector 105, a low pass filter (LPF) 106, a PID (proportional plus integral plus derivative) controller circuit 107, a switch 108, an attenuator 109, an adder #1 (110), a tunable filter 114, and a driving circuit 115 for driving the tunable filter 114.
The principle of the wavelength locking is as follows. A sine wave from the sine wave generator 104 is superimposed on a driving signal for the tunable filter 114, so that the wavelength of the tunable filter 114 is minutely modulated in accordance with the sine wave. In this state, when a slope of the transmission spectrum of the tunable filter 114 is accorded to the reception wavelength by controlling a bias signal from the exterior (a receiver), an intensity of light transmitted through the tunable filter 114 comes to contain the sine wave as a signal component.
The relationship between phases of the sine wave component of the transmitted light intensity and the sine wave of the sine wave generator 104 changes depending on the positional relationship between the wavelength of the tunable filter 114 and the reception wavelength. For example, where it is assumed that there are no elements that invert the signal in the wavelength locking system, the phase relationship between those two signals is in-phase (the same phase) when the wavelength of the tunable filter 114 is on a shorter wavelength side of the reception wavelength. The phase relationship between those two signals is anti-phase (a reverse phase) when the wavelength of the tunable filter 114 is on a longer wavelength side of the reception wavelength. An output of the synchronous detector 105 is positive (plus) when those two input signals are in the in-phase relationship, and the output of the synchronous detector 105 is negative (minus) when those two input signals are in the anti-phase relationship. In order for a low-frequency component of the output signal from the synchronous detector 105 (the low-frequency component is obtained by the LPF 106) to be equal to 0 (zero), the PID controller circuit 107 performs feedback control by adjusting an operation signal (its output) for the driving circuit 115. Thus the wavelength of the tunable filter 114 can be locked at the reception wavelength.
For wavelength locking, it is initially necessary that the wavelength of the tunable filter 114 is set close to the reception wavelength. This operation is conducted by adjusting the bias signal from the exterior (the receiver) under the condition under which the feedback control using the operation signal is stopped by switching a lock ON/OFF signal from the exterior to OFF. After completing this adjustment, the lock ON/OFF signal is switched to ON, and the above-discussed wavelength locking operation is performed.
Such a wavelength locking system is disclosed in "Passively Temperature-Compensated Fiber Fabry-Perot Filter And Its Application In Wavelength Division Multiple Access Computer Network", Electronics Letters, vol. 26, No. 25, pp 2122-2123.
The above-discussed prior art wavelength locking system in the optical receiver has, however, the following disadvantage. In the wavelength control system of the optical transmitter, the wavelength of the transmitter is largely varied by the approach operation even during the signal transmission period. Therefore, the wavelength of the tunable filter 114 in the optical receiver is also changed during the wavelength locking operation. In the prior art wavelength locking system, the wavelength of the tunable filter 114 after being locked is shifted by the operation signal supplied from the feedback control system including the synchronous detector 105, the PID controller circuit 107 and so forth. Hence, an increase in a gain of the feedback control system and expansion of an output range of the operation signal are needed to increase the amount of the wavelength shift of the tunable filter 114 after being locked.
FIG. 3 illustrates the above situation. This is an example in which the system disclosed in the above-described Japanese Patent Application Laid-Open 8-163092 is used as the wavelength control system of the optical receiver. In FIG. 3, there are two approach periods #1 and #2 between stationary state periods #1 and #2. Changes over time of the operation signal and the wavelength of the tunable filter during those periods are shown. The operation signal increases as the wavelength of the tunable filter goes up.
In the feedback control system, however, the magnitude of the gain has a relationship of trade-off with control precision or accuracy, and the expansion of the output range of the operation signal inevitably leads to an increase in probability of losing the locking and a hindrance to improvement in response characteristics.